An electrophotographic liquid developer is generally composed of a coloring agent such as carbon black and other various pigments; a coating material adsorbed onto or coating on the coloring agent for controlling the electrostatic charge of the toner particles and the dispersion of the toner particles, and further imparting a property of fixing images after development to the toner particles; a dispersing agent which is dissolved in or swelled with a carrier liquid for improving the dispersion stability of the toner particles; a charge controlling agent for controlling the electrostatic charge of the toner particles; and a carrier liquid having a high electric resistance (10.sup.9 to 10.sup.15 .OMEGA..multidot.cm).
For producing such a liquid developer, the following method is generally employed.
First, a coloring agent is mixed with a resin as a coating material and they are melt-kneaded by means of a kneading machine such as a Banbury mixer, an extruder, a kneader, a three-roll mill, etc., at a temperature of higher than the softening point of the coating material to provide a solid mixture. Alternatively, a coloring agent and a coating material is dispersed under kneading in a compatible solvent by means of a dispersing device such as a ball mill, an attritor, etc. The kneaded mixture is then dried or added to a nonsolvent to provide a solid mixture.
The solid mixture thus-obtained is dry-ground by means of a grinder and then dispersed in a small amount of a liquid having a similar electric resistance to that of a carrying liquid together with a dispersing agent by a dispersing machine, such as a ball mill, a paint shaker, a sand mill, etc., to provide a condensed toner liquid. The condensed toner liquid is added to a carrier liquid containing a charge controlling agent to provide an electrophotographic liquid developer.
However, it has now been found that when an ethylenic copolymer is used as a coating material, it is very difficult to disperse the ground solid mixture containing the copolymer by the method described above. Hence, a dispersion capable of being practically used for an electrophotographic liquid developer has not heretofore been obtainable.
As a method of dispersing an ethylenic copolymer, there has been proposed a plasticizing method of using a nonpolar solvent, e.g., in JP-A- No. 61-180248 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"). According to this method, a mixture of an ethylenic copolymer, a pigment, and a nonpolar solvent is heated to a temperature of higher than the temperature capable of making the copolymer soluble, cooled, solidified, and then coarsely ground. Thereafter, the ground mixture is dispersed in a nonpolar solvent using a media type dispersing device.
In this method the cohesive force of the copolymer chain is weakened by plasticizing the copolymer for making the copolymer easily dispersible.
However, the dispersion degree of the dispersion obtained by the aforesaid method is insufficient.
For improving the dispersibility of the dispersion, a method using a polar solvent is described in U.S. Pat. No. 4,631,244. However, this method is not always preferable in charging characteristics for an electrophotographic liquid developer using an electrically insulating liquid as the carrier liquid.
Also, the dispersibility may be improved through mechanical means, but the application of large impact force or shearing force is disadvantageous since this method causes the release of the coloring agent from the covered copolymer and the occurrence of denaturing of the copolymer or the coloring agent itself, which results in changing the fundamental characteristics of the dispersion. Furthermore, such a method increases the production costs of the liquid developer. Thus, the aforesaid method is not preferred for producing an electrophotographic liquid developer.
As described above, an ethylenic copolymer is only slightly dispersible in solvent by ordinary methods, owing to the cohesive energy of the copolymer itself. Hence, when an electrophotographic liquid developer is prepared using an ethylenic copolymer as a coating material, it frequently happens that images of good quality are not obtained caused by the inferiority in dispersibility.